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Herein, we present the first example of Os-catalyzed efficient reductive amination under water–gas shift reaction conditions. The developed catalytic systems are formed in situ in aqueous solutions, employ as small as 0.0625 mol% osmium and are capable of delivering reductive amination products for a broad range of aliphatic and aromatic carbonyl compounds and amines. The scope of the reaction, active catalytic systems, possible limitations of the method and DFT-supported mechanistic considerations are discussed in detail in the manuscript.

Thirteen new metallacarborane complexes of rhodium and iridium with covalently bound cage carbon atoms were synthesized and their thermal stability was investigated. Two iridium complexes undergo a polyhedral rearrangement with the formation of more than one isomer. The structures of the new isomers were determined by a single crystal X-ray diffraction analysis and 11B{1H}–11B{1H} COSY NMR. A full isomerization scheme of the less thermally stable complex was proposed based on DFT calculations. According to this mechanism sequential downhill and uphill bifurcations arise in the reaction pathway. Each bifurcation is responsible for a new product formation.

Reaction of the thallium salt Tl[7,8-Me2-10-SMe2-7,8-nido-C2B9H8] with [(cod)RhCl]2 in THF furnishes new rhodacarborane 1,2-Me2-3,3-(cod)-8-SMe2-3,1,2-closo-RhC2B9H8 (1). The latter undergoes 1,2 → 1,7 carbon atom isomerization upon reflux in o-xylene for 5 h producing complex 1,8-Me2-2,2-(cod)-11-SMe2-2,1,8-closo-RhC2B9H8 (2). Reaction of complex 2 with 1 equiv of I2 in benzene results in a loss of the cod ligand and formation of product 3, consisting of cationic [(1,8-Me2-11-SMe2-closo-2,1,8-RhC2B9H8)2I3]+ (3a+) and anionic [(1,8-Me2-11-SMe-closo-2,1,8-RhC2B9H8)2I3]− (3b–) iodide species. In contrast, unisomerized analog 3,3-(cod)-8-SMe2-closo-3,1,2-RhC2B9H10 reacts with I2 affording dimeric iodide [3,3-I2-8-SMe2-closo-3,1,2-RhC2B9H10]2 (5). The structures of 2 and 5 were determined by a single crystal X-ray diffraction.

Osmium derivatives are not as popular among catalytic chemists as the compounds of iron (an analog of osmium), copper, or manganese. Although osmium is expensive, poisonous and volatile, it finds applications not only in cis-hydroxylation of olefins but its derivatives have recently been employed in the oxygenation of C–H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides. The turnover in alkane oxidations catalyzed by soluble osmium derivatives has been found to be much higher than analogous reactions that used complexes of other transition metals (e.g., iron, manganese or copper). Using certain additives, the authors of this review have developed osmium-containing systems that are extremely efficient in transforming alkanes into corresponding alkyl hydroperoxides, which can further be reduced to alcohols easily. The new catalytic systems described here are capable of producing ground-breaking results in the field of metal-complex catalysts.

An efficient method for the CF3-carbenoid C-H functionalization of 6-arylpurines has been developed. This protocol uses readily available methyl 3,3,3-trifluoro-2-diazopropionate as a cross-coupling partner and proceeds smoothly under chelation-controlled Rh(iii) catalysis. The reactions provide the corresponding carbene insertion products with high regioselectivity within a few hours and allow the introduction of both the CF3 and carboxylate functions into biologically important purine molecules including nucleoside derivatives.

The reactions of thallium salt Tl[7-SMe2-7,8-C2B9H10] with [(cod)RhCl]2 and [Cp*RuCl]4 are accompanied by room temperature polyhedral rearrangement giving rhoda- and ruthenacarboranes 1-cod-8-SMe2-1,2,8-RhC2B9H10 (2) and 1-Cp*-8-SMe2-1,2,8-RuC2B9H10 (3). According to DFT-calculations, the rearrangement could be attained to the Triangular Face Rotation (TFR) mechanism. The reaction of iridium derivative 1-cod-8-SMe2-1,2,8-IrC2B9H10 (1) with anhydrous hydrohalic acids HX (X = Cl, Br, I) results in the dimeric halide iridacarboranes [1,1-X2-8-SMe2-1,2,8-IrC2B9H10]2 (4a-c, X = Cl (a), Br (b), I (c)). Bromide 4b reacts with Tl[Tl(η-7,8-C2B9H11)] and TlCp giving iridacarboranes 8'-SMe2-1,1'-Ir(2,3-C2B9H11)(2',8'-C2B9H10) (5) and 1-Cp-8-SMe-1,2,8-IrC2B9H10 (6). The formation of latter compound is accompanied by demethylation of the SMe2 substituent. Structures of 2, 3, 5 and 6 were determined by X-ray diffraction. Iridacarboranes 4b and [1,1-Br2-4-SMe2-1,2,3-IrC2B9H10]2 (7) catalyze the dimerization of diphenylacetylene giving 1,2,3-triphenylnaphthalene in 20-24% yields. 4b and 7 also effectively catalyze the reductive amination reaction between aldehydes (or ketones) and primary (or secondary) amines in the presence of carbon monoxide giving the corresponding secondary and tertiary amines in high yields (60-85%).

Demethylation of cyclohexadienyl complexes (η-X-SMe2-7,8-C2B9H10)Fe(η5-C6H7) (1a,b; X = 9 (a), 10 (b), where X is number of the substituent position) with PhCH2SNa in DMF (dimethylformamide) and subsequent protonation by acetic acid lead to iron benzene complexes (η-X-SMe-7,8-C2B9H10)Fe(η-C6H6) (2a,b). Visible light irradiation of 2a in the presence of tBuNC or Cp*Fe(η-cyclo-P5) affords neutral half-sandwich (η-9-SMe-7,8-C2B9H10)Fe(tBuNC)3 (3) or triple-decker (η-9-SMe-7,8-C2B9H10)Fe(μ-η:η-cyclo-P5)FeCp* (4) complexes. Reaction of 2b with (THF)W(CO)5 selectively gives the iron-tungsten dinuclear complex (η-10-SMe(W(CO)5)-7,8-C2B9H10)Fe(η-C6H6) (5). Structures of 2a, 4 and 5 were determined by X-ray diffraction. Electrochemistry reveals that the redox processes of the SMe-substituted ferracarboranes are cathodically shifted (by 350 mV) with respect to the corresponding redox changes of the SMe2-analogs. The Fe-C6H6 bonding in 2a and the related benzene complexes [(η-9-SMe2-7,8-C2B9H10)Fe(η-C6H6)]+ and (η-7,8-C2B9H10)Fe(η-C6H6) was analyzed by energy decomposition analysis.

Polyhedral rearrangement of iridacarborane 1,2-Me2-3,3-(cod)-4-SMe2-3,1,2-closo-IrC2B9H8 (1) proceeds in a solution at RT and affords a product of 1,2 → 1,7 isomerization 1,8-Me2-2,2-(cod)-7-SMe2-2,1,8-closo-IrC2B9H8 (2). Rhodium derivative 1,2-Me2-3,3-(cod)-4-SMe2-3,1,2-closo-RhC2B9H8 (3) is significantly more stable toward heating than iridium complex 1 and isomerizes at 110 °C by 1,2 → 1,2 and 1,2 → 1,7 reaction schemes with 1,2 → 1,2 being the predominant route forming 1,2-Me2-4,4-(cod)-8-SMe2-4,1,2-closo-RhC2B9H8 (4) and 1,8-Me2-2,2-(cod)-7-SMe2-2,1,8-closo-RhC2B9H8 (5). Complexes 4 and 5 were characterized by 11B{1H}–11B{1H} COSY NMR spectrometry. A mechanism of 1,2 → 1,2 isomerization of 3 to 4 was proposed on the basis of DFT calculations. Reaction of the thallium salt Tl[7,8-Me2-9-SMe2-7,8-nido-C2B9H8] (CarbTl) with [Cp*RuCl]4 in THF furnishes new ruthenacarborane 1,2-Me2-3-(Cp*)-4-SMe2-3,1,2-closo-RuC2B9H8 (6). Heating of 6 at 80 and 144 °C leads to the partial and complete decomposition, respec...

Metallacarborane [1,8-Me2-2,2-(C2H4)2-7-SMe2-2,1,8-IrC2B9H8] (1) was obtained by an interaction of [(C2H4)2IrCl]2 with Tl[9-SMe2-7,8-Me2-7,8-C2B9H8]. Reaction proceeds with a low temperature “1,2 → 1,7” polyhedral rearrangement. Ethene complex 1 reacts with iodine in toluene with the formation of ionic compound 3, consisting of cationic [(1,8-Me2-7-SMe2-2,1,8-IrC2B9H8)2I3]+ (3a+) and anionic [(1,8-Me2-7-SMe-2,1,8-IrC2B9H8)2I3]− (3b–) iodide species. The structures of 1 and 3 were determined by X-ray diffraction.

A systematic study of the osmium-catalyzed reductive amination reaction using carbon monoxide as a reducing agent is presented. Seven cationic, anionic, and neutral osmium complexes with various ligands (including Cl−, arene, Cp*, CO, and pyridine) were synthesized and tested under different conditions. [(cymene)OsCl2]2 (0.5 %) in the presence of a silver salt was found to be optimal. Pleasingly, the developed methodology could be applied to aliphatic and aromatic aldehydes and ketones as well as primary and secondary amines allowing the preparation of various secondary and tertiary amines in good to excellent yields with an average TON of 77. The catalytic performance of the reaction was compared with that of previously reported iridium-, ruthenium-, and rhodium-catalyzed reactions, and the following trend was observed: Ir

The Copper complexes with 1,10-phenanthrolines and 1,10- phenanthrolines-5,6-diones were synthesized and characterized by elemental analysis and spectra. One complex was characterized by X-ray analysis. The complexes turned out to be active in oxidations of cyclic, branched and linear alkanes. Hydrogen peroxide (50%) was used as an oxidant. The reactions were carried out in solvent acetonitrile at relatively the low temperatures (40–60 °C). Alkyl hydroperoxides are the primary products which partly decompose in the course of the reaction to produce mainly ketones (aldehydes) and alcohols. Treating the reaction mixture with PPh3 leads to the transformation of alkyl hydroperoxides (ROOH) to corresponding alcohols (ROH).

The review covers the synthesis, structural features and catalytic activity of rhoda- and iridacarborane halide complexes containing monoanionic dicarbollide [L-C2B9H10]− (L = SMe2, NMe3) and tricarbollide [tBuHN-C3B8H10]− ligands. These complexes are structural analogs of the well-known cyclopentadienyl complexes [Cp*MCl2]2, which are widely used as catalysts for various C–H activation processes. Although the search for applications of halide metallacarboranes started not a long time ago, they have already proved to be efficient catalysts for oxidative coupling of benzoic acids with internal alkynes and reductive amination of carbonyl compounds without external hydrogen source. The influence of substituents in the carborane cage on the structure and catalytic activity is also discussed.

An efficient method for the CF3-carbenoid C-H functionalization of 6-arylpurines has been developed. This protocol uses readily available methyl 3,3,3-trifluoro-2-diazopropionate as a cross-coupling partner and proceeds smoothly under chelation-controlled Rh(iii) catalysis. The reactions provide the corresponding carbene insertion products with high regioselectivity within a few hours and allow the introduction of both the CF3 and carboxylate functions into biologically important purine molecules including nucleoside derivatives.

Room temperature reactions of the rhodium and iridium bromides (η-9-SMe2-7,8-Me2-7,8-C2B9H8)MBr2 (M = Rh (1a), Ir (1b)) with CpTl give the dicarbollide thioether complexes (η-9-SMe-7,8-Me2-7,8-C2B9H8)MCp (2a,b), as a result of demethylation of the SMe2 substituent. These reactions are accompanied by transformation of cage structure from pseudocloso to closo. Refluxing of the sulfonium derivatives [(η-9-SMe2-7,8-Me2-7,8-C2B9H8)MCp]+ (3a,b) with [Bu4N]Br also affords 2a,b. In the same manner ruthenacarboranes [(η-9-SMe2-7,8-C2B9H10)Ru(η-C6H6)]+ (4a) and [(η-10-SMe2-7,8-C2B9H10)Ru(η-C6H6)]+ (4b) react with halide ions giving the thioether complexes (η-9-SMe-7,8-C2B9H10)Ru(η-C6H6) (5a) and (η-10-SMe-7,8-C2B9H10)Ru(η-C6H6) (5b). Dependence of the demethylation rate on the nature of halide ion and solvent was studied both experimentally and theoretically. The structures of 2a and 3aPF6 were determined by X-ray diffraction.

The efficient (turnover numbers attained 1200) oxidation of alkanes to the corresponding alkyl hydroperoxides by H2O2 in the presence of catalytic amounts of ferrocene proceeds in MeCN at 40–50 °C. Benzene is oxidized in the same system to phenol. An obligatory component of the catalytic system for both reactions is pyrazine-2-carboxylic acid (PCA) or trifluoroacetic acid (TFA). Kinetic study as well as selectivity parameters testified that the oxidation proceeds with the participation of hydroxyl radicals. In the case of PCA as a co-catalyst the initial rate W0 of the reaction with both cyclohexane and benzene depends quadratically on [Cp2Fe]0 whereas the two reactions in the presence of TFA are of half order in ferrocene. The ferrocene-catalyzed reaction of cyclohexane with H 2 O 2 16 in an atmosphere of labeled 18O2 gave after 2 h a mixture of labeled and unlabeled cyclohexyl hydroperoxide (total yield 20% based on starting cyclohexane) containing up to 69% of 18O (the analysis was after reduction of cyclohexyl hydroperoxide into cyclohexanol with PPh3).

Two new osmium(II) complexes containing π-coordinated p-cymene (cym) ligands, [(η6-p-cym)Os(bipy)(MeCN)](PF6)2 (1) and (η6-p-cym)Os(py)Cl2 (2) where bipy is 2,2′-bipyridine, have been synthesized. Both complexes efficiently catalyze oxygenation of alkanes (cyclohexane, n-octane, methylcyclohexane, cis- and trans-isomers of 1,2-dimethylcyclohexane), benzene and 1-phenylethanol with hydrogen peroxide in acetonitrile solution in air at 60 °C. Alkanes are oxidized predominantly to alkyl hydroperoxides which can be easily reduced by PPh3 to afford corresponding alcohols. In the cyclohexane oxygenation catalyzed by complex 2 used in a very low concentration (2 × 10−7 M) turnover number (TON) attained 83,000 after 24 h; initial turnover frequency (TOF) was 3400 h−1. The ESI-MS spectrum of the reaction mixture demonstrated the absence of the initial dication [(η6-p-cym)Os(bipy)(MeCN)]2+ which indicates that complex 1 is only a pre-catalyst. Benzene and 1-phenylethanol were transformed into phenol and acetophenone, respectively. The regio- and bond-selectivity parameters measured for the oxidation of linear and branched alkanes and a kinetic analysis of the dependence of initial cyclohexane oxidation rate on concentration of the alkane indicates that the reaction proceeds via attack of hydroxyl radicals at a substrate. The half order of the reaction rate in respect to osmium concentration [Os] can be explained by the assumption that an osmium compound Os introduced into the reaction mixture is rapidly dimerized Os + Os ⇌ D with the equilibrium constant K1. The catalytic activity of the system is due to a monomeric osmium species. The monomer Os forms an adduct with a pyridine molecule (constant K9): Os + py ⇌ Os·py. The interaction with H2O2 affords hydroxyl radicals with rate constant k10. The obtained kinetic data allowed us to estimate values K1/K92 = 520 M and k10 = 1 M−1 s−1.

Isomeric cage-like tetracopper(II) silsesquioxane complexes [(PhSiO1.5)12(CuO)4(NaO0.5)4] (1a), [(PhSiO1.5)6(CuO)4(NaO0.5)4(PhSiO1.5)6] (1b) and binuclear complex [(PhSiO1.5)10(CuO)2(NaO0.5)2] (2) have been studied by various methods. These compounds can be considered as models of some multinuclear copper-containing enzymes. Compounds 1a and 2 are good pre-catalysts for the alkane oxygenation with hydrogen peroxide in air in an acetonitrile solution. Thus, the 1a-catalyzed reaction with cyclohexane at 60 °C gave mainly cyclohexyl hydroperoxide in 17% yield (turnover number, TON, was 190 after 230 min and initial turnover frequency, TOF, was 100 h−1). The alkyl hydroperoxide partly decomposes in the course of the reaction to afford the corresponding ketone and alcohol. The effective activation energy for the cyclohexane oxygenation catalyzed by compounds 1a and 2 is 16 ± 2 and 17 ± 2 kcal mol−1, respectively. Selectivity parameters measured in the oxidation of linear and branched alkanes and the kinetic analysis revealed that the oxidizing species in the reaction is the hydroxyl radical. The analysis of the dependence of the initial reaction rate on the initial concentration of cyclohexane led to a conclusion that hydroxyl radicals attack the cyclohexane molecules in proximity to the copper reaction centers. The oxidations of saturated hydrocarbons with tert-butylhydroperoxide (TBHP) catalyzed by complexes 1a and 2 exhibit unusual selectivity parameters which are due to the steric hindrance created by bulky silsesquioxane ligands surrounding copper reactive centers. Thus, the methylene groups in n-octane have different reactivities: the regioselectivity parameter for the oxidation with TBHP catalyzed by 1a is 1 : 10.5 : 8 : 7. Furthermore, in the oxidation of methylcyclohexane the position 2 relative to the methyl group of this substrate is noticeably less reactive than the corresponding positions 3 and 4. Finally, the oxidation of trans-1,2-dimethylcyclohexane with TBHP catalyzed by complexes 1a and 2 proceeds stereoselectively with the inversion of configuration. The 1a-catalyzed reaction of cyclohexane with H216O2 in an atmosphere of 18O2 gives cyclohexyl hydroperoxide containing up to 50% of 18O. The small amount of cyclohexanone, produced along with cyclohexyl hydroperoxide, is 18O-free and is generated apparently via a mechanism which does not include hydroxyl radicals and incorporation of molecular oxygen from the atmosphere.

The soluble osmium complexes containing p-cymene (π-p-cym) ligands, [(η6-p-cym)OsCl2]2 (1), [(η6-p-cym)Os(bipy)Cl]PF6 (2), and [(η6-p-cym)2Os2(μ-H)3]PF6 (3), are efficient catalysts for the oxidation of alkanes (cyclohexane, n-heptane, methylcyclohexane, isooctane, and cis- and trans-1,2-dimethylcyclohexane) with hydrogen peroxide in air to the corresponding alkyl hydroperoxides in acetonitrile solution if a small amount of pyridine is present in the solution. The binuclear complex 1 is the most active precatalyst in the oxidation whereas compound 2 containing the bipyridine ligand is much less efficient. The oxidation of cyclohexane at 60 °C and low concentration [1]0 = 10−7 M gave a turnover number (TON) of 200 200 after 24 h. A study of the selectivity parameters in the oxidation of linear and branched alkanes and the kinetic peculiarities of the cyclohexane oxidation led to the conclusion that the main reaction mechanism includes the formation of hydroxyl radicals. The effective activation energy Ea for the cyclohexane oxidation catalyzed by complex 1 was 10 ± 2 kcal mol−1. A kinetic analysis verified also that monomerization of complex 1 occurs before the oxidizing species is involved in the catalytic cycle. The 1-catalyzed reaction of cyclohexane, c-C6H12, with H216O2 in an atmosphere of 18O2 gave labeled cyclohexyl hydroperoxide, c-C6H11–18O–18OH. In addition, a small amount of “light” cyclohexanone, c-C6H1016O, is produced apparently via a mechanism which includes neither hydroxyl radicals nor incorporation of molecular oxygen from the atmosphere. The oxidation of benzene with H216O2 under 18O2 gave phenol which did not contain the 18O isotope. The reactions with cyclohexane and benzene were shown to proceed also via an alternative minor mechanism with oxo derivatives of high-valent osmium “OsO” as key oxidizing species.

Reaction of (η-9-SMe2-7,8-Me2-7,8-C2B9H8)M(cod) with hydrohalic acids affords the 16-valence-electron complexes (η-9-SMe2-7,8-Me2-7,8-C2B9H8)MX2 (M = Rh, X = Cl (3a), Br (3b), I (3c); M = Ir, X = Cl (4a), Br (4b), I (4c)). According to X-ray analysis, 3a, 3b, 4b, and 4c have monomeric pseudocloso-structures with a long cage C⋯C distance (2.122, 1.985, 2.238, and 2.186 A, respectively), in contrast to dimeric closo-structures of the cage-non-methylated analogs. The structural difference was explained using DFT calculations. 3b and 4b react with SMe2 and Tl[Tl(η-7,8-C2B9H11)] giving closo-metallacarboranes (η-9-SMe2-7,8-Me2-7,8-C2B9H8)M(SMe2)Br2 (M = Rh (5), Ir (6)) and (η-9-SMe2-7,8-Me2-7,8-C2B9H8)M(η-7,8-C2B9H11) (M = Rh (7), Ir (8)).

Visible light irradiation of the dicarbollide complex [(η-9-SMe2-7,8-C2B9H10)Fe(η-C6H6)]+ (2a) in the presence of the benzene derivatives in CH2Cl2/MeNO2 resulted in cations [(η-9-SMe2-7,8-C2B9H10)Fe(η-C6R6)]+ (2b-g; arene is anisole (b), toluene (c), m-xylene (d), mesitylene (e), durene (f), and hexamethylbenzene (g)) due to the arene exchange. The structures of [2g]PF6 and related tricarbollide complex [(η-1-ButNH-1,7,9-C3B8H10)Fe-(η-C6H6)]PF6 (1) were confirmed by X-ray diffraction analysis. The nature of bonding in cations 1, 2a, and [CpFe(η-C6H6)]+ was analyzed by an energy decomposition analysis.

The reaction of [7-SMe2-7,8-C2B9H10]− with [(cod)IrCl]2 at room temperature affords iridacarborane 1-cod-8-SMe2-1,2,8-IrC2B9H10. The metal insertion is accompanied by polyhedral rearrangement of the carborane ligand with migration of the substituted carbon atom from the metal bonded pentagonal face. The electronic factors rather than steric ones play the main role in the ligand rearrangement. According to DFT calculations the final iridacarborane is more stable by 6–7 kcal mol− 1 than the transient pseudocloso-1-cod-2-SMe2-1,2,3-IrC2B9H10 complex. Structures of 7-SMe2-7,8-C2B9H11 and 2 were determined by X-ray diffraction.

Visible light irradiation of the benzene complex [(η-1-ButNH-1,7,9-C3B8H10)Fe(η-C6H6)]+ in the presence of the charge-compensated carborane anions [9-L-7,8-C2B9H10]− (L = SMe2, NMe3) affords ferracarboranes (η-1-ButNH-1,7,9-C3B8H10)Fe(η-9-L-7,8-C2B9H10). Their structures were established by X-ray diffraction analysis.

Visible light irradiation of cations [(Carb)Fe(η-C6H6)]+ in the presence of Cp*Fe(η-cyclo-P5) affords the μ-pentaphospholyl triple-decker complexes [(Carb)Fe(μ-η:η-cyclo-P5)FeCp*]+ (Carb = 9-SMe2-7,8-C2B9H10 (3a) and 1-t-BuNH-1,7,9-C3B8H10 (3b)). Structures of 3a- and 3b[Co(η-7,8-C2B9H11)2] were determined by X-ray diffraction. Bonding in 3a, 3b and mononuclear building blocks was analyzed by energy decomposition analysis.

The benzene complex [1-(η-C6H6)-12-ButNH-1,2,4,12-FeC3B8H10]+ (3a) was synthesized by the photochemical reaction of [(η5-C6H7)Fe(η-C6H6)]+ (1) with the anion [7-ButNH-7,8,9-C3B8H10]− followed by the treatment of ferracarborane 1-(η5-C6H7)-12-ButNH-1,2,4,12-FeC3B8H10 (2) with hydrochloric acid. The benzene ligand in cation 3a is replaced by alkyl-substituted benzenes under visible light irradiation in CH2Cl2 to form [1-(η-C6R6)-12-ButNH-1,2,4,12-FeC3B8H10]+ (3b–e; C6R6 is toluene (b), mesitylene (c), hexamethylbenzene (d), or anisole (e)). The structure of [3c]PF6 was established by X-ray diffraction.

The reaction of the iodide complex [(η5-C9H2Me5)RhI2]2 (1) or the acetonitrile complex [(η5-C9H2Me5)Rh(MeCN)3]2+ with Tl[Tl(η-7,8-C2B9H11)] afforded rhodacarborane (η5-C9H2Me5)Rh(7,8-C2B9H11) (2). The cationic triple-decker complex with the bridging boratabenzene ligand [Cp*Fe(μ-η:η-C5H3Me2BMe)Rh(η5-C9H2Me5)]2+ (3) was synthesized by the reaction of the nitromethane solvate [(η5-C9H2Me5)Rh(MeNO2)3]2+ with the sandwich compound Cp*Fe(η-C5H3Me2BMe). The structure of 2 was established by X-ray diffraction.

The visible light irradiation of the [(η5-C6H7)Fe(η-C6H6)]+ cation (1) in acetonitrile resulted in the substitution of the benzene ligand to form the labile acetonitrile species [(η5-C6H7)Fe(MeCN)3]+ (2). The reaction of 1 with ButNC in MeCN produced the stable isonitrile complex [(η5-C6H7)Fe(ButNC)3]+ (3). The photochemical reaction of cation 1 with pentaphosphaferrocene Cp*Fe(η-cyclo-P5) afforded the triple-decker cation with the bridging pentaphospholyl ligand, [(η5-C6H7)Fe(μ-η:η-cyclo-P5)FeCp*]+ (4). The latter complex was also synthesized by the reaction of cation 2 with Cp*Fe(η-cyclo-P5). The structure of the complex [3]PF6 was established by X-ray diffraction.

The reduction behavior of the isoelectronic complexes [CpMIII(η6-C6R6)]2+ (M=Rh, Ir; R=H, Me) and [(η-9-SMe2-7,8-C2B9H10)MIII(η6-C6R6)]2+ (M=Rh, Ir; C6R6 = C6H6, C6H5OMe, C6H3Me3) has been studied by cyclic voltammetry and controlled potential coulometry in acetonitrile and propylene carbonate at 253 and 298 K, respectively. The extent of chemical reversibility of the pertinent sequences Rh(III)/Rh(II)/Rh(I) and Ir(III)/Ir(I) is highly dependent on both the nature of the solvent and the intrinsic electronic properties of the arene substituents. The arene η6 coordination makes the derivatives in their lower oxidation states notably short lived, even if, in some cases, the use of propylene carbonate improves their stability or causes the increase in their lifetimes before changing the arene coordination from η6 to η4. Cations [(η-9-SMe2-7,8-C2B9H10)M(η6-C6R6)]2+ were obtained by the bromide abstraction from [(η-9-SMe2-7,8-C2B9H10)MBr2]2 with Ag+ in the presence of benzene and its derivatives. The structure of [(η-9-SMe2-7,8-C2B9H10)Ir(η6-C6H5OMe)](BF4)2 was determined by X-ray diffraction.

The reaction of the [(η-9-SMe2-7,8-C2B9H10)IrBr2]2 complex with Tl[Tl(η-7,8-C2B9H11)] afforded the iridacarborane compound (η-9-SMe2-7,8-C2B9H10)Ir(η-7,8-C2B9H11). The cationic complex [Cp*Ir(η-9-SMe2-7,8-C2B9H10)]+PF6− (5 · PF6, Cp* is pentamethylcyclopentadienyl) was synthesized by the reaction of [Cp*IrCl2]2 with Na[9-SMe2-7,8-C2B9H10]. The structures of (η-9-SMe2-7,8-C2B9H10)Ir(η-cod) (cod is 1,5-cyclooctadiene) and 5 · PF6 were established by X-ray diffraction.

The dicationic arene complexes [CpM(arene)](BF4)2 (arene = C6H6, 1,3,5-C6H3Me3, or C6Me6) were synthesized by the reactions of the solvated complexes [CpM(MeNO2)3](BF4)2 (M = Rh, Ir) with benzene and its derivatives. The solvated complexes were generated in situ by abstraction of I− from [CpMI2]2 with AgBF4. A procedure was developed for the synthesis of the iodide [CpRhI2]2 based on the reaction of the cyclooctadiene derivative CpRh(1,5-C8H12) with I2. The structure of the [CpRh(C6Me6)](BF4)2 complex was established by X-ray diffraction analysis.

The photochemical reaction of [(η5-C6H7)Fe(η-C6H6)]+ (1) with the [(η-9-SMe2-7,8-C2B9H10)]− anion followed by the treatment of the resulting ferracarborane (η-9-SMe2-7,8-C2B9H10)Fe(η5-C6H7) (2) with hydrochloric acid afforded the benzene complex [(η-9-SMe2-7,8-C2B9H10)Fe(η-C6H6)]+ (3). The reaction of cation 3 with ButNC produced the isonitrile complex [(η-9-SMe2-7,8-C2B9H10)Fe(tBuNC)3]+ (4). The structure of the complex [4]PF6 was established by X-ray diffraction.

The benzene complexes [(η-9-L-7,8-C2B9R2H8)Fe(η-C6H6)]+ (3a: L = SMe2, R = H; 3b: L = SMe2, R = Me; 3c: L = NMe3, R = H) were prepared by photochemical reaction of [(η5-C6H7)Fe(η-C6H6)]+ with carbo...

Visible light irradiation of cation [(η5-C6H7)Fe(η-C6H6)]+ (1+) in acetonitrile results in substitution of the benzene ligand giving the labile acetonitrile derivative [(η5-C6H7)Fe(MeCN)3]+ (2a+). The stable isonitrile and phosphite complexes [(η5-C6H7)FeL3]+ [L = tBuNC (2b+), P(OMe)3 (2c+), P(OEt)3 (2d+)] were obtained by reaction of 1 with L in MeCN. The structures of 2cPF6, [CpFe(η-C6H6)]PF6 (3PF6), and Cp∗Fe(η-C6H6)]PF6 (4PF6) were determined by X-ray diffraction. The redox activity of the cyclohexadienyl complexes 1+, 2b+−2d+ has been investigated by electrochemical techniques and compared with that of the related cyclopentadienyl complexes 3+ and 4+. DFT calculations of the redox potentials and the respective geometrical changes were performed. Variable temperature Mossbauer (ME) spectroscopy has elucidated the relationship between structure and formal oxidation state of the iron atom in these complexes. In the case of 3+ an unexpected pair of crystallographic changes has been observed and interpreted in terms of both a second and first order phase transition. The mean-square-amplitude-of-vibration of the metal atom has been compared between the ME and X-ray data. ME measurements in a magnetic field have shown that in 4+ the quadrupole splitting is positive as it is in ferrocene.

The reaction of (η5-C9H2Me5)Rh(1,5-C8H12) (1) with I2 gives the iodide complex [(η5-C9H2Me5)RhI2]2 (2). The solvate complex [(η5- C9H2Me5)Rh(MeNO2)3]2+ (generated in situ by treatment of 2 with Ag+ in nitromethane) reacts with benzene and its derivatives giving the dicationic arene complexes [(η5-9H2Me5)Rh(arene)]2+ [arene = C6H6 (3a), C6Me6 (3b), C6H5OMe (3c)]. Similar reaction with the borole sandwich compound CpRh(η5-C4H4BPh) results in the arene-type complex [CpRh(μ-η5:η6-C4H4BPh)Rh(η5-C9H2Me5)]2+ (4). Treatment of 2 with CpTl in acetonitrile affords cation [(η5-C9H2Me5)RhCp]+ (5). The structure of [3c](BF4)2 was determined by X-ray diffraction. The electrochemical behaviour of complexes prepared was studied. The rhodium–benzene bonding in series of the related complexes [(ring)Rh(C6H6)]2+ (ring = Cp, Cp∗, C9H7, C9H2Me5) was analyzed using energy and charge decomposition schemes.